(1) Field of the Invention
This invention relates to the deposition of thin films of metal on substrates, and more particularly to the vapor deposition of a thin metallic film utilizing an ionized gas arc directed onto a target material spaced from a substrate to be coated in a substantial vacuum while providing a pressure differential between the target and the substrate so that as a portion of the target is vaporized the vapors are carried to the substrate.
(2) Description of Related Art
Vapor deposition for coating a thin film of material on a substrate is known in the art and applications of this process have increased dramatically over the past few years. Applications for such coatings have grown substantially and this includes the electronics industry. Electronic requirements for different resistivity vary from zero for superconductors to infinity for transistor separation. Wide variations in value for thermal conductivity has also resulted in various applications. Other important properties which can be affected by such coating include reflectivy and/or transmission of different wavelengths of energy, corrosion resistance, stability, absorption, hardness, and bond strength. Other applications for vapor deposited coatings are being developed rapidly.
Although there are many processes that are utilized for vapor deposition, each process has inherent characteristics that are advantageous for certain particular applications including the coating of irregular shaped bodies. Some of the vapor deposition processes commonly used today include plasma-activated chemical vapor deposition, laser chemical vapor deposition, sputtering, cathode-spot arc coating, electron beam evaporation deposition, ion plating, arc evaporation and cathode arc plasma deposition. However, all of the known processes tend to have relatively slow deposition rates compared to non-vapor coating methods. Most vapor deposition processes are driven by vapor pressure differentials toward the surface being coated. Lower pressure of other gases in the system tends to increase the mass flow of the coating vapors and results in fewer obstructions of the vapor flow to the surface being coated. Electromagnetic and electrostatic fields are often used to direct the vapor flow and to improve or select the desired particle size utilized in producing the coating.
Many metal joining processes for fabrication, joining sub-assemblies, maintenance and repair in space have been considered by the National Aeronautics and Space Administration. One of these processes utilizes a modified gas tungsten arc welding (GTAW) process including a hollow welding electrode wherein an inert gas is fed through the hollow electrode so as to weld in a space vacuum environment. An arc may be thus established between the electrode and the workpiece as a result of the ionized gas at the electrode so that a GTAW type weld can be made in a vacuum. During this work, vapor deposits were observed.